Method and apparatus for receiving information transmitted through railroad track circuits

ABSTRACT

A method and apparatus for receiving coded information transmitted through track circuits in a railroad block system to a vehicle moving along the railroad track. Successive track circuits encountered by the vehicle provide alternatingly different excitation frequency signals. Each excitation frequency is modulated by a given number of modulation frequencies each representing information to be transmitted. The modulation frequencies are very low to provide a sufficiently high modulation index. A receiver on the vehicle includes a detector for extracting the modulation frequencies from the exciting frequency, frequency multipliers for multiplying the frequency of the detected modulation frequencies, band-pass filters for separating the different multiplied frequencies and indicating devices associated with the filters. The receiver distinguishes between excitation frequencies from adjacent track circuits. Similar fixed receivers may be provided in each track circuit.

United States Patent 1191 Alt July 3, 1973 METHOD AND APPARATUS FOR3,127,476 3/1964 David 179/1555 R RECEIVING INFORMATION 2,953,644 9/1960Miller..... 179/ 15.55 R 3,383,595 /1968 Obata 325/64 TRANSMITTEDTHROUGH RAILROAD 2,133,640 10/1938 Thompson 340/48 TRACK CIRCUITS [75]Inventor: Gilbert L; Alt, Bois-Colombes, Primary Examiner-Albert .l.Mayer France Anomey-Woodhams, Blanchard & Flynn [73] Assignee: Compagnlede Signaux et dEntreprises Electriques, Paris, [57] ABSTRACT France Amethod and apparatus for receiving coded informa- [22] Filed: Jan. 5,1971 tion transmitted through track circuits in a railroad block systemto a vehicle moving along the railroad [21] Appl' 1041l4 track.Successive track circuits encountered by the vehicle providealternatingly different excitation fre- Foreign Application PriorityData quency signals. Each excitation frequency is modulated Jan. 7, 1970France 7000325 by a given number 0f modulatim frequencies eachrepresenting information to be transmitted. The modu- 52 us. c1 325/51,179/82, 246/8, lation frequencies are very low to Pmvide a Sufficiemly325/55 325 3 325 4 329 11 340 47 high modulation index. A receiver onthe vehicle in- 51 161. C1. H04b 1/00 dudes a detect for extracting themdulatin [58] Field 61 Search 325/51, 55, 63, 64, quencies from theexciting frequency frequency 325 351 2; 329/ 111 142; 340 47 4 pliersfor multiplying the frequency of the detected 224; 246/6, 7, 23, 27, 29,30 8; 179/1555 R, modulation frequencies, band-pass filters for separat-1 82 ing the different multiplied frequencies and indicating devicesassociated with the filters. The receiver distin- [56] References (medguishes between excitation frequencies from adjacent UNITEDSTATESPATENTS track circuits. Similar fixed receivers may be provided ineach track circuit. 3,378,817 ,4/1968 Vittm' 325/55 2,117,739 5/1938Miller 179/1555 R 7 Claims, 2 Drawing Figures 15311007455 musk,

BANDPASS FHLTERS TUNED rNPuT MuLTIPLIERS LJ". DEMOD.

F. MULT.

METHOD AND APPARATUS FOR RECEIVING INFORMATION TRANSMITTED THROUGHRAILROAD TRACK CIRCUITS The present invention relates to a method ofcoding track circuits and permitting the transmission of information toa vehicle moving along a railway track which is divided into asuccession of blocks each provided with a track circuit, said successivetrack circuits being excited by different frequencies which are repeatedalong the whole track and the value or magnitude of which frequencies iscontrolled by the characteristics of these circuits, a method in whicheach of the excitation frequencies of the track circuits is modulated bya given number of modulation frequencies each representing informationto be transmitted. The invention also relates to receivers for puttingthis method into practice.

It is known that the safety and regularity of railway traffic moving onrailway tracks are ensured by signals spaced along the tracks andserving to transmit the information which is to bereceived by the driverof the train. At the present time these signals are generally controlledautomatically by safety devices which are known under the name of trackcircuits and ensure safety by means of the block system.

Block systems have long been used to control railway traffic. A blocksystem has been defined as a system by which a track is divided intoshort sections, or blocks, for example, of three or four miles lengthand trains are so run by signal apparatus (block signals) that no trainenters a block until the preceding train has left it.

Nowadays there are trains the speed of which is such that the driver hasonly a short amount of time to notice a given signal and this increasesthe risk of accidents. In addition, with even faster trains which are atpresent under survey, for example turbine trains, it is absolutelyimpossible to envisage using the visual signals which are placed alongtracks.

Therefore, it has been considered necessary to perfect a system whichmakes it possible to transmit directly to the interior of the cabin ofthe driver the information which is necessary to improve the running ofthe train. Such a system must naturally be devised so as not tointerfere with the working of the track circuits which must continue intheir function with complete safety.

Consideration has therefore been given to modulating the excitationfrequencies of the track circuits with a given number of modulationfrequencies each representing information to be transmitted, then todetecting these modulation frequencies directly on the train as well asat the end of each track circuit.

However, the value or magnitude of the excitation frequencies of thetrack circuits is controlled by the characteristics of said circuits. Ifnormal modulation frequencies capable of being separated by means ofconventional filters are'used, the modulation index is extremely weakand the greatest difficulty is then experienced in detecting thesemodulation frequencies with an adequate margin of safety.

Naturally lower modulation frequencies could be used, but then thefilters capable of separating such frequencies present difficulties.

The aim of the present invention is to eliminate these disadvantagesand, for this purpose, it provides a method of the above-mentioned typewhich is essentially characterized in that is consists of using very lowmodulation frequencies so that the modulation index is sufficientlyincreased to detect these different modulation frequencies on the trainand at the end of each track circuit, and then multiplying theirfrequency by a given factor in order to obtain frequencies which can beeasily separated by means of conventional band-pass filters.

It can be easily understood that a relatively simple and economicalmethod of transmission is obtained in this way, said method alsoensuring complete safety which is required particularly by railways. Inparticular, the use of a track circuit modulated in frequency providesthe system with a remarkable immunity from the eddy currents produced byelectric tractions. It also appears that the decoding of the informationcorresponding to different modulation frequencies makes it possible todifferentiate the information received at the end of each track circuitand therefore to avoid the connections which are usually produced bycable in the block system. Therefore, everything concerning thereception and decoding of information on the machine likewise applies tothe reception of-this information at the end of each track circuit. In aparticular application of the invention, the excitation frequencies ofthe track circuits are measured in kilohertz and the modulationfrequencies in 10 hertz, whereas the multiplication factor is equal tothe power of 2 the exponent of which is at least equal to 4.

Such a multiplication factor makes it possible to obtain safety andfrequencies of more than 200 Hz, i.e., frequencies which can be easilyseparated by means of conventional band-pass filters.

A receiving device for putting into practice the method of transmissionaccording to the invention is characterized in that it comprises adetector for extracting from the collected signal the differentmodulation frequencies, a frequency multiplier for multiplying thefrequency of detected modulation frequencies, and a series of band-passfilters for separating these different multiplied frequencies, eachfilter being associated with an indicating device controlled by thesignal coming from the corresponding filter.

On the machine, the indicating devices comprising for example luminoussignals possibly associated with electric bells, are actuatedselectively as a function of the modulation frequencies which aredetected and therefore directly provides the driver of the train withthe necessary information.

The indicating devices associated with receivers which are positioned onthe track are themselves usually formed by safety relays the position ofwhich corresponds to the permitted speed on each track circuit.

In a preferred embodiment of the invention, the frequeney multipliercomprises a first multiplier stage insensitive to the frequency,followed by a second multiplier stage forming a series of circuits, eachbeing tuned to one of the frequencies which is capable of beingdelivered by the first multiplier stage.

The multiplication factor of the first multiplier stage is preferablyequal to 2, whereas the multiplication factor of the second multiplierstage is equal to 8.

A practical embodiment of the invention is described below by way of anexample, with reference to the accompanying drawings in which FIG. 1 isa synoptic diagram of a receiver for putting into practice the method oftransmission according to the invention. The description will of FIG. 1only concern the receiver positioned on the railway vehicle, thereceivers of each track circuit being identical with the exception ofthe collector which is an element peculiar to the vehicle.

FIG. 2 schematically discloses structure associated with the trackcircuits.

Firstly, it will be recalled that the method of transmission accordingto the invention is intended for the transmission of information to avehicle moving along a railway track which is provided with successivetrack circuits excited by different frequencies. In order to clarify thedescription, it will be assumed that these successive track circuits areexcited by two different frequencies, for example 1,700 Hz and 2,300 Hz,which are repeated alternately along the entire track. I

In accordance with the invention, each of these excitation frequenciesof 1,700 Hz and 2,300 Hz is modulated by a given number of very lowfixed modulation frequencies, each representing a piece of informationto be transmitted to the vehicle. If, for example, there are sevendifferent pieces of information to be transmitted, these modulationfrequencies can be advantageously produced between 12 Hz and 20 Hz. Themod-- ulation index is therefore relatively increased.

More particularly, by long known theory (Set forth, for example, in theelectronic treatise Techniques de IIngem'eur, 1953, installment E 3100,pages l, 3 in the Article Modulation de Frequence Modulation parimpulsions by Pierre Besson), the modulation index k can be expressedas:

I k AF/f,

where F is the carrier (excitation) frequency, AF is the carrierfrequency deviation (increase or decrease in excitation or carrierfrequency) and f is the modulation frequency. Thus, if, for example, theexcitation frequency is increased or decreased by Hz (i.e. AF 10 Hz),there will be a modulation index between 0.5 (f= 20 Hz) and 0.8 (f= 12Hz) To avoid mutual interference with normal functions of the trackcircuits, the deviation AF and pass band is kept low. The modulationfrequenciesfare kept low to provide a sufficiently 'high modulationindex k. As stated, if higher modulation frequencies (e.g.,f= 100 Hz)capable of being separated byconventional filters are used, themodulation index k is extremely weak (e.g., k 10/100 0.1) and difficultyis experienced in detecting these modulation frequencies in the receiverwith reliability.

Moreover, it should be noted that as a result of the modulation, carrierfrequencies of 1,700 Hz and 2,300 Hz no longer appear in the trackcircuits. In fact, with an increase or decrease in frequency equal to 10Hz, either 1,690 Hz or 1,710 Hz will be obtained for example in a giventrack circuit, whereas in two adjacent track circuits a modulatedfrequency of 2,290 Hz or 2,310. Hz will always be obtained.

The different means of modulating, during transmission, the excitationfrequencies of different track circuits according to the invention areall known and will therefore not be described in detail. However,reference may conveniently be made to FIG. 2 which disclosesschematically a track V comprising two rails r and r, along which avehicle, usually a locomotive, A moves, e.g., in direction F. Therailroad track V is providedwith successive track circuits CV,, CV etc.extending between joints .1. The track circuits are excited by the twodifferent frequencies F and F e.g., 1,700 Hz and 2,300 Hz, repeatedalternately along the track, provided by suitable generators E. Theexcitation frequencies F and F are modulated by the low, fixedmodulation frequencies f,,f ,f etc., provided by suitable modulators M,and each representing a piece of information to be transmitted to thevehicle. The different means are associated with a receiver carried bythe vehicle the aim of which is to collect and detect the modulatedfrequencies generated in the successive track circuits in order toextract the information therefrom. Such a receiver is synopti callyrepresented in FIG. 1 and described in detail below. Generally similarreceivers, indicated for example at RF, and RF may be associated withcorresponding track circuits.

This receiver comprises firstly a collector 1 to be positioned on thevehicle, usually a locomotive, in the vi cinity of the railway track onwhich it runs and preferably equidistant from the two rails. In thedescribed embodiment the collector comprises two resonant circuits 2 and3, each being tuned to one of the excitation frequencies of the trackcircuits, 1,700 Hz and 2,300 Hz respectively. These resonant circuitscomprise simply a capacitor C in parallel with an inductance coil Lprovided with an iron core, and they are respectively connected to thein-puts of two amplifiers 4 and 5 which are enclosed in the collector 1.Moreover, each of these amplifiers is tuned to the correspondingfrequency of the associated circuit.

The collector 1 is tuned to the remainder of the receiver which isadvantageously enclosed in a box positioned inside the driving cabin ofthe train, by two leads 6 and 7 connecting the out-puts of the twoamplifiers 4 and 5 to the in-puts of two band-pass filters 8 and 9respectively. These two band-pass filters are respectively tunedtofrequencies of 1,700 Hz and 2,300 Hz and are followed by two amplifiers10 and 11 likewise tuned to these two frequencies.

The amplifier 10 controls the functioning of a first commutating relay12 via a lead 13, whereas the amplified signal appears at an out-putterminal 14(Similarly, the amplifier 11 controls a second commutatingrelay 15 similar to the first via a lead 16 whereas the amplified signalappears at an out-put terminal 17. These two amplifiers l0 and 11 arefed by means of two feed terminals 18 and 19 via a voltage supply +V0.

The relay 12 comprises three contact breakers, 20, 21 and 22. Thecontact breaker 20 which is normally open is positioned between thevoltage supply +Vo and the exciting winding of a delaying relay 23comprising a single contact breaker 24. The contact breaker 21 which islikewise normally open is positioned between the out-put terminal 14 ofthe amplifier 1'0 and the contact breaker 24 of the relay 23. As for thecontact breaker 22, it is not normally closed and located between feedterminal 19 of the amplifier 11 and the voltage supply +Vo.

The relay 15 also comprises three contact breakers, 25, 26 and 27,respectively. The contact breaker 25 which is normally open is locatedbetween the voltage supply +V0 and the exciting winding of the delayingThe working contact of the delaying relay 23 is connected by a lead 28to the in-put of a very low frequency demodulator 29 which is connectedin series with a multiplier '30 which is relatively insensitive tofrequency. This multiplier 30, the multiplication factor of which isequal to 2, is capable of multiplying directly the frequency of avariable frequency signal, i.e., of signals of differing frequencies.

The out-put of the multiplier 30 is simultaneously connected to sevenmultipliers M to M the in-put of each of these circuits being tuned toone of the seven frequencies capable of appearing at the out-put of thecircuit 30. These tuned multipliers M, to M are conventional in designand their multiplication factor is equal to 8 The multiplier 30 is ofknown type, for example, the type Doubler of Frequency of M. De Jolydescribed in the Agenda Dunod Electricity, 1942 edition Paris, France,pages 219 and 220 (with FIG. 75). The multiplier 30 likewise may besimilar to multipliers Ml-M7 but with greater band width.

Each of the multipliers M, to M is connected in series to a band-passfilter, F, to F respectively, the output signal of which controlstheoperation of a relay R to R Each of these seven relays R, to R, isassociated with a contact breaker such as 31 and selectively insures thefunctioning of seven indicating devices V, to V comprising for exampleluminous signals. For this purpose, each of the signals V, to V isconnected between the voltage supply +V0 and the earth by means of thecontact breaker 31 of one of the relays R, to R The receiver which hasjust been described operates as follows:

It should first be assumed that at a given moment the collector 1attached to the locomotive is located above a portion of the trackcircuit excited by a signal having a frequency of 1,700 Hz. in thiscase, the tuned circuit 2 picks up the signal and transmits it to theamplifier 4. it should be remembered at this point that the collectedsignal is not actually a signal of 1,700 Hz, but a modulated signalhaving alternatively frequencies of 1,690 Hz and 1,710 Hz, in harmonywith the modulation signal in use which represents the information to betransmitted to the locomotive. Moreover, it should be remembered that inaccordance with the invention this modulation signal has a very lowfrequency between 12 and 20 Hz.

The amplified signal appearing at the out-put of the amplifier 4 istherefore transmitted by the lead 6 to the band-pass filter 8. Thisband-pass filter is tuned to a frequency of 1,700 Hz and its aim is toeliminate the harmonics which may have been generated. The filteredsignal is then transmitted to the amplifier 10 which is fed with currentfrom the constant voltage supply +V0 via the closed contact 27 of therelay 15.

The amplifier 10 is designed in such a manner that when it receives asignal at its in-put, it causes excitation of the commutating relay 12by way of the lead 13, which causes the contact breaker 22 to open andthe contact breakers and 21 to close.

The opening of the contact breaker 22 cuts off the current supply to theamplifier 11 which is therefore rendered inoperative. Consequently, nosignal can be transmitted by the second track which comprises the tunedcircuit 3, the amplifier 5, the filter 9 and the amplifier l1, and whichis reserved for frequencies of 2,300 Hz.

The simultaneous closing of the contact breaker 20 causes the current tobe fed to the delaying relay 23,

the contact 24 of which is closed with a given time lag of a second. Themodulated signal at the out-put terminal 14 of the amplifier 10 istherefore transmitted to the demodulator 29 by means of the closedcontacts 21 and 24 and the lead 28. The delayed-response relay 23therefore delays transmission of the collect signal at the demodulator29, so that it is possible to avoid the transmission of transientphenomena which occur when the collector 1 passes over the cross-oversbetween the successive track circuits. When the collector 1 passes overa track circuit excited by a signal having a frequency of 2,300 Hz, thetuned circuit 3 accepts the modulated signal and transmits it to theamplifier 5. This modulated signal has alternate frequencies of 2,290 Hzand 2,310 Hz, in harmony with the modulation signal in use at that time.

On the other hand, the tuned circuit 2 no longer picks up any signalbecause it is tuned to 1,700 Hz. The commutating relay 12 thereforereturns to its initial state, which causes currentto be fed to theamplifier 11 by way of the closed contact 22. As this amplifier 11receives at its in-put the collected and modulated signal, via theamplifier 5 and. the band-pass filter 9, it causes the commutating relay15 to be excited by the lead 16 which in turn causes the contact 27 toopen and the contacts 25 and 26 to close.

, The opening of the contact 27 cuts off the current being fed throughthe amplifier 10 of the 1,700 Hz track, thereby rendering said trackinoperative whereas the closing of the contact 25 effects the feeding ofthe delayed-response relay 23. When the contact 24 of this relay isclosed, the modulated signal at the out-put terminal 17 of the amplifier11 is transmitted to the in-put of the de-modulator 29.

Therefore, when one of the tracks, for example that which is reservedfor 1,700 Hz, is in operation the other track is rendered inoperativeand vice versa, so that interference between the two tracks can beavoided. In addition, the delayed-response relay 23 causes the modulatedsignal to be transmitted with a certain time lag to the demodulator 29in order to avoid transient phenomena occurring at the cross-oversbetween the different track' circuits.

The demodulated signal appears at the output of the demodulator 29 inthe form of a succession of pulses the recurring frequency of whichcorresponds, at a given time, to the modulation frequency in use at thesame time. This demodulated signal is therefore a variable frequencysignal. In this particular embodiment being described, it is capable ofproviding seven different frequencies between l2 and 20 Hz, each of saidfrequencies representing one of the seven pieces of information to betransmitted to the driver of the train.

The frequency of the demodulated signal collected at the output of thedemodulator 29 is first multiplied by two in the multiplier 30. Thiscircuit 30 is insensitive to the frequency of the input signal.Therefore, at its output a signal is collected which is always ofvariable frequency' but the frequency of which is doubled, i.e., asignal capable of providing seven different frequencies between 24 and40 Hz. This double frequency signal is then applied to the input of theseven tuned multipliers M to M the multiplication factor of'which isequal to eight. It follows that the seven frequencies at the output ofthe different multipliers M to M, are produced between 192 Hz and 320Hz, which makes it possible for them to be filtered by means ofband-pass filters F, to F each being tuned to one of said sevenfrequencies.

When one of the filters F, to F receives a frequency corresponding toits tuning frequency, the associated relay is excited and its contact 31closed by causing one of the signals V to V, to be lit, which enablesthedriver of the train to receive the corresponding information.

It should be assumed for example that at a given time the collector 1picks up a signal modulated in frequency by 12 Hz, corresponding togiven information which is to be transmitted to the driver of the train.The frequency of the demodulated signal collected at the output of thedemodulator 29 is therefore equal to 12 Hz, which gives a frequency of24 Hz at the output of the circuit 30. This frequency of 24 Hz is thenmultiplied by eight in the multipliers M to M the input of which istuned to a frequency of 24 Hz, for example the circuit M The filter Ftherefore receives at its input a frequency of 192 Hz correspondingprecisely to its tuning frequency, which causes excitation of theassociated relay R the contact 31 of which is closed.Consequently, thesignal V is lit up and therefore indicates to the driver of the trainthe information corresponding to the modulation frequency of 12 Hz. Whenthe modulation frequency changes, another signal is lit and the lightingof each signal corresponds therefore to a specific piece of information.

Therefore the driver directly receives inside the driving cabin of thetrain the different information which is requiredito ensure the goodrunning and complete safety of the train. ln this particular examplebeing described, each of these pieces of information is manifested bythe lighting of a luminous signal, but it is obvious that numerousalternative indicating devices could be used.

At the end of each track circuit, the information is decoded by means ofa receiver similar to the one described above, and a relay peculiar toeach indication can be excited. The number of transmitted pieces ofinformation is only limited by the value of the coefficient ofmodulation and caneasily be of the order of to 30.

It is noted moreover that the information furnished by this method ofcoding the track circuits can likewise be used independently of thetrack mechanism relation when it is simply desired to increase theselectivity of the receivers of the track circuits in order to reducemore effectively the stray currents circulating in the rails andparticularly produced by the mechanisms producing strong harmonics ofthe driving current such as the mechanisms of Thyristors and themachines of Choppers or Hacheurs.

In this case, the quantity of necessary information can be greatlyreduced and limited with a single modulation which could thus beadvantageously realized by using a submultiple of the proper frequencyof the track circuit. Beginning with the oscillator furnishing thecarrier frequency, a classic vernier arrangement di viding the frequencyby a fixed number gives the frequency modulation which directlymodulates the signal of the oscillator before amplification and thereceiver proves to be simplified for it is sufficient to detect a singletype of modulation.

What we claim is:

l. A method of coding track circuits and permitting the transmission ofinformation to a vehicle moving along a railway track divided into asuccession of block sections each provided with a track circuit, saidsuccessive track circuits being excited by different frequencies whichare repeated along the entire track and the value of which frequenciesis determined by the characteristics of said track circuits, the methodcomprising the steps of modulating each of the exciting frequencies ofthe track circuit by a specific number ofmodulation frequencies, eachmodulation frequency representing a piece of information to betransmitted, the modulation frequencies being very low in order that themodulation index may be sufficiently increased to enable detection ofthese different modulation frequencies onthe railway vehicle and at theend of each track circuit; detecting said modulation frequencies; thenmultiplying the frequency of the detected modulation frequencies by agiven factor in order to obtain frequencies which can be easilyseparated by means of conventional band-pass filters; and using suchfilters to separate said multiplied modulation frequencies.

2. A method as claimed in claim 1, characterized in that the excitingfrequencies of the track circuits are of the order of a kilohertz andthe modulation frequencies are measured in 10 hertz, whereas themultiplication factor is equal to the power of 2 the exponent of whichis at least equal to 4.

3. ln apparatus for use in coding track circuits and permitting thetransmission of information to a vehicle moving along a railway trackdivided into a succession of block sections each provided with a trackcircuit, successive track circuits being excited by differentfrequencies repeated along the entire track and the value of whichfrequencies is determined bythe characteristics of said track circuits,each of the exciting frequencies of the track circuit being modulated bya specific number of modulation frequencies each representing a piece ofinformation to be transmitted, the modulation frequencies being very lowin order that the modulation index may be sufficiently increased toallow detection of these different modulation frequencies on the railwayvehicle and .at the end of each track circuit, a receiver comprising: ademodulator for extracting the different modulation frequencies from thecollected signal to provide demodulated frequencies, a frequencymultiplier following saiddemodulator for multiplying the frequency ofsaid demodulated frequencies to provide different multipliedfrequencies, and a plurality of band-pass filters following saidfrequency multiplier to separate said different multiplied frequencies,and an indicating device associated with each said band-pass filtercontrolled by the signal coming from the corresponding filter.

4. A receiver as claimed in claim 3, characterized in that the frequencymultiplier comprises a first multiplying stage which is insensitive tothe frequency, followed by a second multiplying stage which is formed bya plurality of multiplier circuits, each being tuned to a different oneof the frequencies which can be delivered by the first multiplyingstage.

5. A receiver as claimed in claim 4, characterized in that themultiplication factor of the first multiplier stage is equal to 2,whereas the multiplication factor of the second stage is equal to 8.

6. A receiver as claimed in claim 3, including collector means having apair of resonant circuits each tuned to a corresponding one of theexcitation frequencies for receiving from the track circuitscorresponding modu- 9 lator excitation frequencies in alternation as thevehicle passes from block section to block section, commutating relaysand means connecting ones of said commutating relays to correspondingones of said resonant circuits for actuation thereby, means responsiveto actuation of one said commutating relay by one said resonant circuitfor preventing actuation of'the other conimutating relay from the otherresonant circuit, means responsive to actuation of a one of saidcommutating relays for providing a time delay connection .of the inputof said demodulator to said means connected to a corresponding one ofsaid resonant circuits.

7. A receiver as claimed in claim 6, in which each said means connectedto a resonant circuit comprises a band-pass filter tuned to thecorresponding exciting frequency for eliminating spurious harmonics andan amplifier connected between such band-pass filter and thecorresponding commutating relay for driving such commutating relay, eachsuch commutating relay including a contact for shutting off the supplyof operating potential to amplifiers connected to other resonantcircuits soas to positively prevent application of spurious signals tosaid demodulator from resonant circuits other than those tuned to theparticular track circuit along which the vehicle is passing.

1. A method of coding track circuits and permitting the transmission ofinformation to a vehicle moving along a railway track divided into asuccession of block sections each provided with a track circuit, saidsuccessive track circuits being excited by different frequencies whichare repeated along the entire track and the value of which frequenciesis determined by the characteristics of said track circuits, the methodcomprising the steps of modulating each of the exciting frequencies ofthe track circuit by a specific number of modulation frequencies, eachmodulation frequency representing a piece of information to betransmitted, the modulation frequencies being very low in order that themodulation index may be sufficiently increased to enable detection ofthese different modulation frequencies on the railway vehicle and at theend of each track circuit; detecting said modulation frequencies; thenmultiplying the frequency of the detected modulation frequencies by agiven factor in order to obtain frequencies which can be easilyseparated by means of cOnventional band-pass filters; and using suchfilters to separate said multiplied modulation frequencies.
 2. A methodas claimed in claim 1, characterized in which the exciting frequenciesof the track circuits are of the order of a kilohertz and the modulationfrequencies are measured in 10 hertz, whereas the multiplication factoris equal to the power of 2 the exponent of which is at least equal to 4.3. In apparatus for use in coding track circuits and permitting thetransmission of information to a vehicle moving along a railway trackdivided into a succession of block sections each provided with a trackcircuit, successive track circuits being excited by differentfrequencies repeated along the entire track and the value of whichfrequencies is determined by the characteristics of said track circuits,each of the exciting frequencies of the track circuit being modulated bya specific number of modulation frequencies each representing a piece ofinformation to be transmitted, the modulation frequencies being very lowin order that the modulation index may be sufficiently increased toallow detection of these different modulation frequencies on the railwayvehicle and at the end of each track circuit, a receiver comprising: ademodulator for extracting the different modulation frequencies from thecollected signal to provide demodulated frequencies, a frequencymultiplier following said demodulator for multiplying the frequency ofsaid demodulated frequencies to provide different multipliedfrequencies, and a plurality of band-pass filters following saidfrequency multiplier to separate said different multiplied frequencies,and an indicating device associated with each said band-pass filtercontrolled by the signal coming from the corresponding filter.
 4. Areceiver as claimed in claim 3, in which the frequency multipliercomprises a first multiplying stage which is insensitive to thefrequency, followed by a second multiplying stage which is formed by aplurality of multiplier circuits, each being tuned to a different one ofthe frequencies which can be delivered by the first multiplying stage.5. A receiver as claimed in claim 4, in which the multiplication factorof the first multiplier stage is equal to 2, whereas the multiplicationfactor of the second stage is equal to
 8. 6. A receiver as claimed inclaim 3, including collector means having a pair of resonant circuitseach tuned to a corresponding one of the exciting frequencies forreceiving from the track circuits corresponding modulated excitingfrequencies in alternation as the vehicle passes from block section toblock section, commutating relays and means connecting ones of saidcommutating relays to corresponding ones of said resonant circuits foractuation thereby, means responsive to actuation of one said commutatingrelay by one said resonant circuit for preventing actuation of the othercommutating relay from the other resonant circuit, means responsive toactuation of a one of said commutating relays for providing a time delayconnection of the input of said demodulator to said means connected to acorresponding one of said resonant circuits.
 7. A receiver as claimed inclaim 6, in which each said means connected to a resonant circuitcomprises a band-pass filter tuned to the corresponding excitingfrequency for eliminating spurious harmonics and an amplifier connectedbetween such band-pass filter and the corresponding commutating relayfor driving such commutating relay, each such commutating relayincluding a contact for shutting off the supply of operating potentialto amplifiers connected to other resonant circuits so as to positivelyprevent application of spurious signals to said demodulator fromresonant circuits other than those tuned to the particular track circuitalong which the vehicle is passing.